In crystal oscillator circuits, a crystal can be driven at its resonant frequency to generate a high quality periodic signal. In such circuits it can be desirable to regulate the amplitude of the oscillator drive signal. Such amplitude regulation can increase the lifetime of the circuit. In addition, regulation can reduce the likelihood of harmonic mode oscillation, which can introduce higher, unwanted frequencies into the oscillator signal.
To better understand various features of the disclosed embodiments, a previously known amplitude control circuit will now be described. An amplitude control circuit is shown in FIG. 4, and designated by the general reference character 400. An amplitude control circuit 400 (or amplitude control loop ACL) can include an input section 402, a conversion section 404, an output drive section 406, and filter section 408. An input section 402 can generate a peak detection voltage representing a peak level of input signals inp and inn. In addition, input section 402 can generate an average current based on such signals.
A peak voltage value can be stored by a capacitor C41 at node 410. Such a value can be applied, via conversion section 404, to output drive section 406. This can generate an output signal at node 412. Such a signal can result in the generation of an output current for controlling an amplifier that drives a crystal.
A circuit like that of FIG. 4 is shown in co-pending provisional patent application Ser. No. 60/506,265, titled AMPLITUDE CONTROL FOR CRYSTAL OSCILLATOR, by Mike McMenamy, filed Sep. 25, 2003 and nonprovisional patent application Ser. No. 10/949,902, filed Sep. 24, 2004, having the same title and inventor.
A drawback to an arrangement like that of FIG. 4 can be that at lower supply voltages, such a circuit may not provide a correct amplitude control.